Brain metastases pose a significant problem for women with advanced metastatic diseases. The rate of brain metastasis has increased significantly in the last 10 years, approaching or exceeding 35% in subpopulations of metastatic breast cancer patients, particularly those with Her2+ or triple-negative tumors. After diagnosis of multiple metastatic lesions, patients typically die within one to two years. Gap: Treatment of brain metastases is primarily palliative due to limited curative effectiveness of radiation, surger, and poor delivery of chemotherapy across the blood-brain barrier (BBB). This proposal focuses on preventing metastasis seeding and initial growth in brain using preclinical models. Hypothesis: By reducing the efficiency of metastatic cancer cell penetration into brain and increasing drug delivery and efficacy in early micrometastatic lesions, we will decrease large metastases development and improve both neurological function and overall survival. Aim 1: Demonstrate that brain invasion of metastatic breast cancer cells can be inhibited at the level of the BBB: Preliminary data indicate TGF- inhibition reduces brain invasion, at the level of the BBB, of triple negative human metastatic breast cancer cells in vivo ~70-80%. This work is extended to characterize mechanisms of how TGF- inhibition reduces BBB cell invasion and the role Her2+ plays in BBB invasion. Aim 2: Elucidate the causal relationship between blood-tumor barrier permeability changes and chemotherapeutic uptake and effect in brain micrometastases of breast cancer: Preliminary data suggest that, contrary to common assumptions, most micrometastatic lesions (<500 m diameter) of breast cancer in brain show marked changes in metastatic vasculature structure and function, including vessel co- option, reduced vascular density, enhanced permeability, and elevated VEGF expression. In this Aim, work will be completed to characterize BBB changes in micrometastases, with the goal of identifying selective difference in micrometastases, such as VEGF and Notch, which can be used for targeted therapeutic benefit. Aim 3: Develop novel strategies to modulate blood-brain barrier permeability to improve therapeutic efficacy for brain micrometastases treatment: Preliminary data demonstrate that inhibition of vascular endothelial Notch signaling in both large and small brain metastases in the presence of VEGF results in targeted increases in BBB permeability. In this Aim, the targeted increases in permeability are exploited to improve chemotherapy delivery, cytotoxic effect and overall survival in three preclinical tumor models. Impact: The goal of this proposal is to develop novel approaches to reduce breast cancer cell invasion to brain, to modulate BBB permeability and improve chemotherapy uptake into CNS metastases, with an overall purpose to reduce brain metastases related death. The work requires state-of-the-art cell targeting, BBB permeability, and drug distribution methods to link barrier changes in selected small tumors with overall brain metastasis invasion and growth.